The present invention relates to a liquid temperature control apparatus for controlling to a constant level the temperature of an ink ejected from a head of, for example, an ink jet printing apparatus.
In an ink jet printer, an ink supplied from an ink reservoir or tank is usually heated by a heater mounted on an ink supply tube, delivered to an ejection head via the tube, and then ejected from a nozzle of the head to form a string of ink droplets. The size of an ink droplet is largely dependent on the ink temperature and a change in the droplet size has critical influence on the quality of a pattern to be reproduced on a recording medium. With this in view, an ink jet printer is furnished with a liquid temperature control apparatus which controls the temperature of ink directed to the head to a predetermined level. Such an apparatus usually comprises a temperature sensor held in contact with the head to sense an ink temperature inside the head, and a controller for energizing the heater in accordance with an output of the temperature sensor.
A drawback inherent in a prior art liquid temperature control apparatus of the type described is that, since the heater mounted on the tube is spaced a substantial distance from the head, the sensor cannot sense an ink temperature without a delay which corresponds to a period of time consumed by the ink to flow through the distance up to the head. Because the flow rate of the ink is as small as 1 cc/min, several tens of seconds are necessary for the head to be heated up to a predetermined level by the hot incoming ink, so that a substantial period of time is consumed before the sensor senses a temperature elevation to the predetermined level. Accordingly, in case where the controller 18 is of the high speed response type, the ink temperature within the head tends to rise to an unusual level or even to a boiling point. Such a high ink temperature may burn the heater 20 or deteriorate the property of the ink. The sensor mounted in contact with the head may result a subtle change in the orientation of the head which is reflected by a change in the direction of ink ejection or an intricate construction of the ink jet printer.
These drawbacks may be eliminated by slowing down the response of the controller or regulating a voltage applied to the heater, as has been conventionally practiced. However, this accompanies a shortcoming that the temperature building of the ink ejected from the head is noticeably decelerated.
Another known type of liquid temperature control apparatus comprises a first temperature sensor located between the ink reservoir and the heater so as to sense an ink temperature before the ink is heated by the heater, a second temperature sensor mounted in contact with the heater, and a controller controlling power supplied to the heater in accordance with a difference between output voltages of the two sensors. This is neither acceptable due to a very slow thermal response which is attributable to the use of a power transistor as the heater and utilization of a collector loss which occurs upon supply of a current to the power transistor.
Such a slow buildup of the heater results in a disproportionate period of time which the ink directed to the head takes to be heated to the predetermined temperature, preventing a printing operation until the temperature becomes stable. This is usually coped with by a small sized design of the heater which speeds up the buildup of the heater. The head is made up of various parts such as a piezoelectric vibrator, a support member and a casing. When the ink jet printer is turned off, the head needs about 2-10 minutes to be cooled off naturally to the ambient level while the heater takes only about 30-60 seconds to be so cooled off. When the ink jet printer is turned on again after a short "off" period and the heater is heated as in the initial start-up (when the ink jet printer is first turned on), the temperature of ink flowing to the head becomes elevated beyond a temperature range permissible for printing operation. Thus, a quite longer waiting period is necessary for the ink flowing to the head to be cooled off to the specific range than in the initial condition of operation. Though this may be eliminated if a temperature sensor is mounted on the head, the sensor in this case will subtly affect the direction of ink ejection from the head due to its mechanical connection with the head by an electric wire. Also, the power transistor is large sized and therefore takes a substantial period of time to be heated. An additional and substantial time period is necessary to heat a member which transmits the temperature to the ink.